Now that winter has arrived I will be doing a lot of riding on the DL-1 with 66 degree seat angle. Saddle clamp behind the post. Putting the clamp in front completely changes character of bike, I just like the way back feel.
When used for local trips there is no problem. Longer rides reliably cause knee issues. It will be behind the knee and more in the tendons than in bone. There is never any acute pain, it sneaks up. Noticed first while walking, not while riding. Then one day stand out of saddle and sit back down real quick. Not because of pain, because it feels like nothing is supporting weight. Finish the ride entirely sitting in saddle.
It always heals fairly quickly. This would be with daily rides of twenty miles (enough in winter on a DL-1) and occasional fifty mile rides. Would not even dream of attempting 300k with the 66 degree seat angle.
The ancients did not all reverse the seatpost. Somewhat more recently Steve Bauer raced Paris-Roubaix and a couple other early season races on a 62 degree bike. Then did the remainder of his last pro season on 67 degrees. All with seatpost in the normal position. That episode was an extreme outlier, still shows that it is not impossible. I would not try it.
Small wheel comfort is even more dependent on tires than it is with big wheels. The Giant looks like ISO 406? There are lots and lots of high performance tires in the BMX world. It is a different world, takes some time to find your way around. The old standby has been the Panaracer HP406. Fast, comfortable, reliable, even lightweight. Wife has them on her Raleigh Twenty, bigger change than any of the other mods.